1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to a phase-shifting mask and a process for fabricating a phase-shifting mask.
2. Discussion of Related Art
Improvements in photolithography have increased the density and enhanced the performance of semiconductor devices by shrinking integrated circuits (ICs). As described by the Rayleigh criterion, the minimum critical dimension (CD) which can be resolved by a wafer stepper is directly proportional to the wavelength of the illumination source and inversely proportional to the numerical aperture (NA) of the projection lens. However, diffraction tends to degrade the aerial image when the CD becomes smaller than the actinic wavelength. The actinic wavelength is the wavelength of light at which a mask is used in a wafer stepper to selectively expose photoresist coated on a substrate, such as a Silicon wafer. As needed, a resolution enhancement technique (RET), such as a phase-shifting mask (PSM), may be used to achieve a wider process latitude. Unlike a binary mask that only uses Chrome to control the amplitude of light transmitted through a quartz substrate, a PSM further modulates the phase of light to take advantage of destructive interference to compensate for the effects of diffraction.
An alternating PSM (AltPSM) is a type of PSM that is particularly helpful in improving contrast when patterning very small CDs, such as the gate length of a transistor in a device. AltPSM introduces a phase shift of 180 degrees between the light transmitted through adjacent clear openings so destructive interference can force the amplitude between the two images to zero. A phase shift of 180 degrees is implemented by creating a difference in the optical path lengths through adjacent openings in an opaque layer, such as Chrome. A subtractive process may be used to etch a trench into the quartz substrate in alternate openings. However, incident light may scatter off the sidewalls and bottom corners of the etched trench and cause an imbalance in the aerial image that varies as a function of focus. Such a waveguide effect may be manifested as a CD error and a placement error.
The intensity and phase in the aerial image of an AltPSM may be balanced in various ways. A selective biasing approach enlarges the CD of the etched opening relative to the unetched opening to balance the aerial image. An etchback approach undercuts the edges of the chrome in both openings to balance the aerial image. A dual-trench approach etches a deep trench in the phase-shifted opening and a shallow trench in the non-phase-shifted opening to balance the aerial image.
Each approach for balancing the aerial image has drawbacks. The selective biasing approach may be limited to the discrete values available on the design grid unless a gray beam-writing scheme is used. The etchback approach may result in defects, such as chipping or delamination of the overhanging chrome between adjacent openings. The dual-trench approach adds complexity and cost by requiring additional processing.
Thus, what is needed is a phase-shifting mask (PSM) having in situ balancing of intensity and phase and a method of forming such a PSM.